The present invention relates to sealable security bags and in particular to providing secure evidence of thermal tampering with a seal.
Tamper-evident sealable security bags are used for storing and transporting valuable goods such as money. The valuables can be sealed into the bag. When the bag comes to be opened the tamper-evident features of the bag indicate to the recipient whether the sealing of the bag has been tampered with. If the tamper-evident features are intact then they give the recipient confidence that the bag's contents have not been interfered with. The recipient can then cut the bag open.
Security bags typically have an enclosed main body formed of sturdy plastics sheet--for example of polythene or polypropylene. The main body has an opening and a flap that can be folded over the opening on to the main body of the bag to close the opening, Either the flap or the area of the main body of the bag which will be in contact with the flap is coated with strong adhesive which, once stuck, provides a strong bond. The strong adhesive and the sturdy plastic sheet make it difficult, but not impossible, for the bag to be opened illegitimately. However, the bag is provided with tamper-evident security features that aim to provide evidence of an attempt to open the bag illegitimately.
One known way to open such a bag illegitimately is to heat the adhesive of the seal to a temperature at which it becomes plastic, but at which the plastics sheet material of which the body of the bag is made is still stable. This temperature is normally above at least 70.degree. C., depending on the adhesive used. At those temperatures the adhesive becomes sufficiently plastic that the bag can be opened, the contents inspected or removed, and then the bag shut. Without any suitable tamper-evident features it would not then be obvious to the recipient that tampering of this sort had occurred. It would thus be impossible to know, for example, if confidential information in the bag had been seen by a third party or to identify where in a supply chain removal of goods from the bag had occurred.
One tamper-evident feature that is known for providing evidence of such thermal tampering is a detailed pattern printed on the outer surface of the flap of the bag, for example by ink-jet printing. When the seal area is heated and the flap comprised from the body of the bag the resultant stretching of the flap becomes evident as a deformation in the detailed pattern. However, it has been argued that because there may potentially be little stretching (since the material of the bag may remain stable at the temperature at which the adhesive softens) R can be difficult for inexperienced users to notice deformation of the pattern.
Another tamper-evident feature that is known for providing evidence of such thermal tampering is a coating on the outer surface of the flap of a heat-sensitive water-based ink. When the seal area is heated the heat-sensitive ink irreversibly changes colour, clearly indicating that the bag has been tampered with. However, because the ink is water-based it can be wiped off the flap to give the appearance that no tampering has occurred.
It would be desirable to provide a seal suitable, for example, for a security bag which addresses the problem of providing obvious and secure evidence of tampering by heating.